perhaps the overlooked text is the part where the gauges are NOT connected to the unit. if this is the case, you must have the gauge hose tightened on the manifold stop with trapped refrigerant. (it's beginning to sound like an abbot and costello routine).
the depressor in the center hose does not prevent freon from flowing through it.
unless you have the gauge hose connected to the unit, you should be reading 0 psig. the only time you will read the tank pressure (normally) is when you are running the system and you are charging vapor into the system through the low side. in that case, the higher pressure you read will be the tank pressure overcoming the suction pressure to force itself in.
by the way, do you know about purging the center hose, and what about the question asked * do you know how much freon to charge into the system * i realize heatseeker is the only one listening here, but it's amazing what people will do without a clue how to do it. there is definitely a time to not do it yourself.